Bonding aid for polyamide resin and method of bonding with the same

ABSTRACT

It is a joining auxiliary agent for a polyamide resin with which a predetermined joining face of a polyamide resin molded article is coated to ensure the joining strength between the predetermined joining face and another polyamide resin when they are joined together, and has a composition containing a compound (1), which cleaves a hydrogen bond in the polyamide resin molded article while assisting the dissolution of the polyamide resin, in an organic solvent capable of dissolving the polyamide resin.

TECHNICAL FIELD

The present invention relates to a joining auxiliary agent with which apredetermined joining face is coated to enhance the joining strength ofa molded article of a polyamide resin represented by nylon (trade name)and a joining method using the same.

BACKGROUND ART

In general, a linear polymer having an amide bond as a repeating unit inthe molecule is referred to as a polyamide. In particular, a polyamidehaving an aliphatic chain as a main chain is widely distributed as aresin with a trade name “nylon”. Typical examples thereof include nylon66, nylon 6 and the like. A polyamide resin is a crystalline polymer, inwhich a hydrogen bond can be formed between amide bonds in differentpolymer chains. The crystal structure thereof is constituted in such amanner that the above-mentioned amide bonds are hydrogen bondedregularly in the same plane and simultaneously, each plane is arrangedin a layered pattern, therefore, it is robust. Because of the polaritydue to this amide bond and the above-mentioned crystal structure, apolyamide resin has an excellent resistance to a hydrocarbon-basedsolvent such as gasoline or oil. In addition, its thermal resistance isrelatively high, and moreover, it has material strength, therefore, itis widely used as a preferred resin material for automobile parts,machine parts and the like. In order to form a part having a shape thatcannot be formed by a single die molding process (e.g., an undercutstructure), it is necessary that the parts with a corresponding shape beformed with a polyamide resin individually. However, in order to obtainstrong joining strength, a method of treating the predetermined joiningface with heat and joining polyamide resins together by close to weldinghas been employed conventionally.

However, for example, by using nylon 6, which is a polyamide resin, whennylon 6 in a melted state, which becomes an additional molded portion,is injected and filled to an existing molded article, which is made ofnylon 6 and is placed in a metal mold, to try to join them together inaccordance with the above-mentioned method, both are joined together,however, the joining strength is not sufficient. In addition, in thecase where they are cooled down without being joined together, they willbe separated in some cases. The cause is as follows. Some of the amidebonds in the polymer chain of nylon 6 forming a predetermined joiningface that is an end surface of the existing molded article are in a freestate in which they do not form a hydrogen bond because there is nopolymer chain, which is an adjacent partner to be mutually hydrogenbonded in the same plane as described above.

In addition, in order to inject and fill a melted resin of nylon 6,which becomes an additional molded portion, in a metal mold, it isnecessary to carry out a preliminary preparation by heating this metalmold and maintaining it at a predetermined temperature. However, theabove-mentioned amide bonds in a free state in the nylon 6 forming thepredetermined joining face of the above-mentioned existing moldedarticle are trying to find a partner for a hydrogen bond for associationas best they can during this heat treatment, and randomly form ahydrogen bond in an amorphous state within the polymer chains in thenylon 6 of their own.

In this way, the amide bonds in the predetermined joining face of theexisting molded article to be hydrogen bonded essentially with the amidebonds in the nylon 6 in a melted state in the additional molded portionare placed in a metal mold in a state in which they already form ahydrogen bond within the polymer chains in the nylon 6 of their own,though they are in an amorphous state. Therefore, the amide bonds in thenylon 6 in a melted state in the additional molded portion to beinjected in the metal mold are mutually hydrogen bonded within thepolymer chains of their own as they are cooled in the metal mold afterthe injection. As a result, the nylon 6 in the existing molded articleand the nylon 6 in the melted resin are crystallized individually,therefore, it is difficult to join them.

The same problem applies to the case where polyamide resin moldedarticles are subjected to ultrasonic welding. More specifically, thevibration energy caused by ultrasonic waves is directed to both of thepredetermined joining faces, and the polyamide resins in the area areslightly melted by heating and mutually hydrogen bonded. Then, beingcooled down, they are supposed to be joined together, however, in theprocess of the heat treatment, each polyamide resin molded article iscrystallized individually as described above. Accordingly, in order tosufficiently weld them by directing the energy more efficiently, theabove-mentioned predetermined joining face is subjected to a treatmentsuch as a precise processing of a number of edges for ensuring the areato be contacted. Both polyamide resin molded articles are joinedtogether, however, it is not sufficient in terms of the joiningstrength.

Incidentally, as a joining method that provides an appropriate joiningstrength by using a polyamide resin molded article, there is aresistance welding method. It is a method for molding of incorporating aconductive electric wire into an article to be molded along the outlineof the article, turning on electricity thereby allowing the conductiveelectric wire to generate heat and to become a hot wire, and whilemelting the article to be molded, injecting and filling a meltedpolyamide resin on the melted article to be molded in the metal mold.However, the efficiency of the operation of incorporating a conductiveelectric wire is very low, which results in a high production cost.

Accordingly, an object of the present invention is to join polyamideresins together with sufficient joining strength.

DISCLOSURE OF THE INVENTION

In order to achieve the above-mentioned object, a gist of the presentinvention described in claim 1 is a joining auxiliary agent for apolyamide resin with which a predetermined joining face of a polyamideresin molded article is coated to ensure the joining strength betweenthe predetermined joining face and another polyamide resin when they arejoined together, in which a compound (1), which cleaves a hydrogen bondin the polyamide resin molded article, while assisting the dissolutionof the polyamide resin, is contained in an organic solvent capable ofdissolving the polyamide resin.

Here, as the organic solvent, alcohols having 1 to 6 carbon atoms,ketones or aldehydes having 1 to 6 carbon atoms, nitriles having 1 to 6carbon atoms are exemplified, and specifically, methanol, isopropylalcohol, acetone, acetonitrile and the like are effective. These organicsolvents are a poor solvent for a polyamide resin which can dissolvevery small amount of a polyamide resin. As the compound (1), monovalentto trivalent phenols and the like are exemplified, it may be a phenolwith another valency.

According to the invention of claim 1, the following effect can beobtained. When a predetermined joining face of a polyamide resin moldedarticle and another polyamide resin are joined, by coating thepredetermined joining face with a joining auxiliary agent of this claim1, while a compound (1) which cleaves a hydrogen bond in the polyamideresin molded article (in the following description, also referred to asmerely “compound (1)”) is cleaving a hydrogen bond between the amidebonds in the adjacent polyamide polymer chains in the predeterminedjoining face, it forms a hydrogen bond with an amide bond whose hydrogenbond has been cleaved or an amide bond in a free state in which it doesnot form a hydrogen bond, whereby it can prevent the amide bonds in thepredetermined joining face from forming a hydrogen bond again. Inaddition, the organic solvent is needed for coating the predeterminedjoining face thinly with the joining auxiliary agent.

Here, the action of the above-mentioned compound (1) to thepredetermined joining face as described above can be confirmed byperforming analysis by the infrared absorption spectra or the like, andit was found that its action is maintained until a heat treatment step.For example, as a method of joining the predetermined joining face of amolded article of a polyamide resin and another polyamide resin whileperforming a heat treatment, there is a method of injecting and fillinga polyamide resin in a melted state of an additional molded portion toan existing molded article of a polyamide resin placed in a metal moldand joining them together. In such a case, the predetermined joiningface of the existing molded article coated with the joining auxiliaryagent can still maintain the activated state in which the hydrogen bondsbetween the amide bonds have been cleaved even after heating in themetal mold. Therefore, the polyamide resin of the existing moldedarticle is in a more amorphous state in the predetermined joining facecompared with the polyamide resin before being coated with the joiningauxiliary agent, whereby it is likely to be in an amorphous statetogether with the amide bonds in the polyamide resin in a melted stateof an additional molded portion to be injected into the metal mold.Then, in the process of cooling in the metal mold after heating,recrystallization is promoted while forming a hydrogen bond between bothresins, whereby it becomes possible to join the polyamide resins with ajoining strength equal to the material strength of an integral article.

A gist of the present invention described in claim 2 is that, in theinvention described in claim 1, a polyamide resin is dissolved andcontained in the above-mentioned joining auxiliary agent. Here, thepolyamide resin is preferably the one which is equivalent to theabove-mentioned polyamide resin molded article.

According to the invention of claim 2, the following effect in additionto the action effect of claim 1 can be obtained. The above-mentionedjoining auxiliary agent loosens the crystal of the polyamide resin ofthe predetermined joining face while slightly dissolving the polyamideresin of the predetermined joining face, and cleaves the hydrogen bondbetween the amide bonds in the polyamide resin, and further promotes thedissolution of the polyamide resin in which the polyamide resin of thedissolved molded article was originally included thereby reducing theoriginal unevenness of the predetermined joining face, whereby thepredetermined joining face can be deformed into a physically smoothsurface by the polyamide resin by itself. Therefore, the area which cancontact the predetermined joining face and the polyamide resin to bejoined in a state close to “plane” increases, whereby the credibility ofthe joining strength can be increased.

A gist of the present invention described in claim 3 is that, in theinvention described in claim 1, a compound (2) which maintains theaction of cleaving a hydrogen bond of the compound (1) which cleaves ahydrogen bond in the above-mentioned polyamide resin molded article inthe above-mentioned joining auxiliary agent. Here, examples of thecompound (2) include organic acids and the like, and specific examplesthereof include formic acid, carboxylic acid and the like, and morespecific examples thereof include monovalent to trivalent carboxylicacid derivatives of phenol, however, it may be a carboxylic acid havinganother valency.

According to the invention of claim 3, the following effect in additionto the action effect of claim 1 can be obtained. In the case where thecompound (1) which cleaves a hydrogen bond in the polyamide resin moldedarticle is a phenol, its anion is stabilized by resonating with thebenzene ring, whereby it is easy to be oxidized or decomposed afterdissociation, therefore, its essential action may be lost. Accordingly,in order to maintain the action of cleaving a hydrogen bond of thecompound (1), a compound (2) exhibiting a smaller pKa than that of thecompound (1) is added, and the compound (2) is made to dissociateitself, whereby the phenol can be prevented from being oxidized ordecomposed, therefore, the action of the compound which cleaves ahydrogen bond in the polyamide resin molded article is not lost.Accordingly, the action of the compound (1) can be maintained, and thecredibility of the joining strength is increased. Incidentally, in thefollowing description, the compound (2) which maintains the action ofcleaving a hydrogen bond of the above-mentioned compound (1) is referredto as merely “compound (2)”

A gist of the present invention described in claim 4 is that, in theinvention described in claim 1, the organic solvent in the joiningauxiliary agent is an organic solvent having a molecular weight of 120or less.

According to the invention of claim 4, the following effect in additionto the action effect of claim 1 can be obtained. An organic solventhaving a molecular weight of 120 or less has a relatively smallmolecule, and can easily come close to the polyamide resin. It overcomesthe intermolecular force and is easy to penetrate into the molecule andmoreover it is easy to evaporate. Therefore, the compound (1) in thejoining auxiliary agent or the dissolved polyamide resin is easy to forma hydrogen bond with the polyamide resin of the predetermined joiningface, whereby the credibility of the joining strength can be increased.

A gist of the present invention described in claim 5 is that, in theinvention described in claim 1, the above-mentioned organic solvent is amixed solvent of organic solvents composed of plural types thereof.

According to the invention of claim 5, the following effect in additionto the action effect of claim 1 can be obtained. By mixing plural typesof organic solvents, such a mixed organic solvent thereof can adjust itto the vapor pressure at which the coating operation of the joiningauxiliary agent is performed without any trouble (compared with the caseof one type of organic solvent). About one minute after coating(assuming that the room temperature is about 20° C.), a solution stateis maintained, therefore, the dissolution of the polyamide resin in themolded article soon after coating is promoted, and formation of ahydrogen bond of the compound (1) or the dissolved polyamide resin andthe polyamide resin of the predetermined joining face proceeds morepromptly.

A gist of the present invention described in claim 6 is that, in theinvention described in claim 1, the above-mentioned compound (1) whichcleaves a hydrogen bond in the polyamide resin molded article is1,3-dihydroxybenzene (CAS number: RN (108-46-3)) or/and3,5-dihydroxybenzenecarboxylic acid (CAS number: RN (99-10-5)).

According to the invention of claim 6, the following effect in additionto the action effect of claim 1 can be obtained. 1,3-dihydroxybenzene(CAS number: RN (108-46-3)) and 3,5-dihydroxybenzenecarboxylic acid havea hydroxyl group or a carboxyl group showing a relatively largepolarity, and can preferably form a hydrogen bond with the amide bond ofa polyamide. In addition, 1,3-dihydroxybenzene (CAS number: RN(108-46-3)) has a sublimation property. 1,3-dihydroxybenzene (CASnumber: RN (108-46-3)) sublimates gradually after coating thepredetermined joining face with the joining auxiliary agent. The thermaldeformation temperature or the molding temperature of, for example, anylon, which is a polyamide resin, is a further higher temperature,therefore, 1,3-dihydroxybenzene almost completely evaporates in theprocess of a heat treatment of the polyamide resin.

On the other hand, 3,5-dihydroxybenzenecarboxylic acid (CAS number: RN(99-10-5)) has a benzene ring and a hydroxyl group in the same manner as1,3-dihydroxybenzene (CAS number: RN (108-46-3)), and has the sameaction effect attributable to these functional groups. However, it is aphenolic carboxylic acid derivative in which a carboxyl group isattached to the meta position of the respective hydroxyl groups. Due tothis characteristic, the pKa value of 3,5-dihydroxybenzenecarboxylicacid (3.0) is smaller than that of 1,3-dihydroxybenzene (9.3), and itdoes not have a sublimation property. Therefore,3,5-dihydroxybenzenecarboxylic acid remains on the joining face afterboth polyamide resins are crystallized, however, it conveniently fillsin the region of noncrystalline or amorphous lattice defect for exertingthe viscosity strength as a polyamide resin material. In addition, if1,3-dihydroxybenzene (CAS number: RN (108-46-3)) and3,5-dihydroxybenzenecarboxylic acid (CAS number: RN (99-10-5)) are usedby appropriately mixing together, they are preferably hydrogen bondedwith the amide bonds of the polyamide.

A gist of the present invention described in claim 7 is that, in theinvention described in claim 2, the above-mentioned polyamide resinmolded article is nylon 6 or nylon 66, and the polyamide resin containedin the above-mentioned joining auxiliary agent is identical to theabove-mentioned polyamide resin molded article.

According to the invention of claim 7, the following effect in additionto the action effect of claim 2 can be obtained. Different from analiphatic ring or an aromatic ring, which has a rigid and symmetric ringstructure, there is flexibility to a certain degree in the configurationof methylene groups of the amide bonded side chains in nylon 6 or nylon66. Therefore, the flexibility of the phase transition from crystal tononcrystal or vice versa is large, and a compound which cleaves ahydrogen bond in the polyamide resin molded article in the joiningauxiliary agent is easy to act on, therefore a sufficient strength canbe obtained. In addition, the polyamide resin contained in theabove-mentioned joining auxiliary agent is also nylon 6 or nylon 66,which is identical to the molded article, therefore, it is easy to beinserted to the predetermined joining face, whereby the credibility ofthe joining strength can be increased.

A gist of the present invention described in claim 8 is that, in theinvention described in claim 3, 1,3-dihydroxybenzene is contained in theabove-mentioned joining auxiliary agent as the compound (1) whichcleaves a hydrogen bond in the polyamide resin molded article, and thecompound (2) which maintains the action of cleaving a hydrogen bond of1,3-dihydroxybenzene (CAS number: RN (108-46-3)) is3,5-dihydroxybenzenecarboxylic acid (CAS number: RN (99-10-5)) and,and/or salicylic acid (CAS number: RN (69-72-7)).

According to the invention of claim 8, the following effect in additionto the action effect of claim 3 can be obtained.3,5-dihydroxybenzenecarboxylic acid suppresses the dissociation of thephenolic hydroxyl group of 1,3-dihydroxybenzene (CAS number: RN(108-46-3)) by dissociating itself in the organic solvent, and a partthereof forms a mutual hydrogen bond with 1,3-dihydroxybenzene (CASnumber: RN (108-46-3)) and maintains the association state with it afterthe organic solvent evaporates. Therefore, it can prevent the oxidationand sublimation of 1,3-dihydroxybenzene (CAS number: RN (108-46-3))simultaneously. Accordingly, the action of 1,3-dihydroxybenzene (CASnumber: RN (108-46-3)), which is the compound (1), is not lost.Furthermore, 3,5-dihydroxybenzenecarboxylic acid (CAS number: RN(99-10-5)) can also act on as the compound (1) with regard to itshydroxyl group region and carboxyl group region, and can enhance theamorphous nature of the predetermined joining face, and moreover, it canmaintain the state.

In addition, the polyamide resin in the joining auxiliary agent can alsoform a hydrogen bond with 1,3-dihydroxybenzene (CAS number: RN(108-46-3)), therefore, it has an action of preventing its sublimation.In this way, it can maintain the quality of the joining auxiliary agentby preventing the oxidation and decomposition of the compound (1) in thejoining auxiliary agent. The organic solvent in the joining auxiliaryagent evaporates soon after being applied, however, 1,3-dihydroxybenzene(CAS number: RN (108-46-3)) remains for a while without evaporating.Furthermore, 3,5-dihydroxybenzene-carboxylic acid (CAS number: RN(99-10-5)) does not evaporate, therefore, the amorphous state of thesurface of the polyamide resin of the predetermined joining face can bemaintained for a long time. Accordingly, even if another polyamide resinto be joined and the polyamide resin of the original molded article tobe joined are not necessarily subjected to a joining treatment soonafter applying the joining auxiliary agent, the joining strength of theboth polyamide resins is not lost, therefore, it can apply to a varietyof joining operation steps.

A gist of the present invention described in claim 9 is that, in theinvention described in claim 4, the above-mentioned organic solvent isat 50% or higher and 90% or lower by weight.

According to the invention of claim 9, the following effect in additionto the action effect of claim 4 can be obtained. If the above-mentionedorganic solvent is at lower than 50% by weight, it is difficult todissolve the compound (1) or the compound (2), and if it is at higherthan 90% by weight, it is difficult to contain the compound (1) or thecompound (2) enough to alter the surface of the polyamide resin.

A gist of the present invention described in claim 10 is that, in theinvention described in claim 6, the above-mentioned compound (1) is at10% or higher and 50% or lower by weight.

According to the invention of claim 10, the following effect in additionto the action effect of claim 6 can be obtained. If the above-mentionedcompound (1) is at lower than 10% by weight, it is difficult to alterthe surface of the polyamide resin enough to be joined, and if it is athigher than 50% by weight, it cannot be dissolved in the above-mentionedorganic solvent.

A gist of the present invention described in claim 11 is that, in theinvention described in claim 7, the above-mentioned polyamide resin isat 0.005% or higher and 1.000% or lower by weight.

According to the invention of claim 11, the following effect in additionto the action effect of claim 7 can be obtained. If the above-mentionedpolyamide resin is at lower than 0.005% by weight, it is difficult tosee the effect at the joining, and if it is at lower than 1.000% byweight, the stability of the strength at the joining becomes excellentin some cases. The actual outcome will be described in the followingExamples.

A gist of the present invention described in claim 12 is that, in theinvention described in claim 5, the above-mentioned organic solventcomprises two types of organic solvents, and the mutual ratio by weightis 0.01 or higher and 100 or lower.

According to the invention of claim 12, the following effect in additionto the action effect of claim 5 can be obtained. If the ratio of theabove-mentioned two types of organic solvents is lower than 0.01 orhigher than 100, it is difficult to see the effect of mixing, however,if it is 0.01 or higher and 100 or lower, it can be adjusted to thevapor pressure at which the coating operation of the joining auxiliaryagent is performed without any trouble.

A gist of the present invention described in claim 13 is that, in theinvention described in claim 8, the mutual ratio by weight of1,3-dihydroxybenzene (CAS number: RN (108-46-3)) and3,5-dihydroxybenzene-carboxylic acid (CAS number: RN (99-10-5)) is 0.001or higher and 1000 or lower.

According to the invention of claim 13, the following effect in additionto the action effect of claim 8 can be obtained. If the mutual ratio of1,3-dihydroxybenzene (CAS number: RN (108-46-3)) and3,5-dihydroxybenzene-carboxylic acid (CAS number: RN (99-10-5)) is lowerthan 0.001 or higher than 1000, it is difficult to see the effect ofmixing, however, if it is 0.001 or higher and 1000 or lower, theoxidation and decomposition of 1,3-dihydroxybenzene can be prevented byadding 3,5-dihydroxybenzenecarboxylic acid which shows a smaller pKathan that of 1,3-dihydroxybenzene thereby allowing it to dissociateitself. Therefore, the action of the compound, which cleaves a hydrogenbond in the polyamide resin molded article, is not lost. Accordingly,the state in which the hydrogen bond in the polyamide resin of thepredetermined joining face has been cleaved can be maintained, and thecredibility of the joining strength is increased.

A gist of the present invention described in claim 14 is that a joiningmethod using a joining auxiliary agent for a polyamide resin in theinvention described in claim 1. Here, as the joining method, aninjection molding joining method in which the joining auxiliary agent ofthis invention is applied to the predetermined joining face of anarticle to be molded, the article to be molded is placed in a metal moldand injection filling is performed, an ultrasonic welding method inwhich joining is performed by providing ultrasonic vibration to apolyamide resin molded article and the like are cited.

According to the invention of claim 14, the following effect in additionto the action effect of claim 1 can be obtained. By adding the joiningmethod of the present invention to various joining methods of apolyamide resin molded article such as an injection molding joiningmethod and an ultrasonic welding method, which are widely used in theindustrial world, a joining strength equal to that of an integral moldedarticle can be obtained. In other words, firm joining of two members byinjection molding, which could not be achieved conventionally, becomespossible. In addition, in the ultrasonic welding method, the joiningarea of welding can be small, therefore, joining can be attained in sucha manner that the joining face has a good appearance. Further, it makesa large contribution to the reduction of cost or reduction of steps inthis technical field. In addition, by using the joining auxiliary agentdescribed in claim 1, a processed product of a polyamide resin withplural members joined firmly together can be formed by a variety ofmolding methods such as blow molding including injection molding andextrusion molding, or ultrasonic welding, vibration welding, etc., oreven by a joining method by heating and melting a joining face such aslaser welding or electromagnetic induction welding. This enables easyproduction at a low cost of a molded article having an undercut shape ora hollow molded article, which could not be formed in a single moldingprocess so far by performing joining.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are perspective views of an existing molded articleportion T₁₁ of a test piece T₁ and the test piece T₁ which has beenjoined by injection molding, respectively.

FIG. 2 is a perspective view of a metal mold M for molding a test pieceT₁ in a separate state.

FIG. 3 is a perspective view of a state in which an existing moldedarticle portion T₁₁ is placed in a metal mold M.

FIG. 4 is a view showing the components of the joining auxiliary agentsof Examples 1 to 11 and the experimental results for a test piece T₁using the respective joining auxiliary agents.

FIG. 5 is a view showing the components of the joining auxiliary agentsof Examples 12 to 22 and the experimental results for a test piece T₁using the respective joining auxiliary agents.

FIG. 6 is a view showing the components of the joining auxiliary agentsof Examples 23 to 33 and the experimental results for a test piece T₁using the respective joining auxiliary agents.

FIG. 7 is a view showing the molding conditions for Comparative Examples1 to 4 and the experimental results.

FIG. 8 is a graph showing the relationship of the joining strength(tensile strength) to the time from coating with a joining auxiliaryagent to completing of the molding in the case where a test piece T₁ isinjected and molded by using the joining auxiliary agents of Example 26and Example 18.

FIG. 9 shows the AFM measurement result of the surface of apredetermined joining face 3 before being coated with a joiningauxiliary agent.

FIG. 10 shows the AFM measurement result of a predetermined joining face3 coated with only methanol in a similar manner.

FIG. 11 shows the AFM measurement result of a predetermined joining face3 five minutes after being coated with a joining auxiliary agent in asimilar manner.

FIG. 12 shows the AFM measurement result of a predetermined joining face3 twenty-four hours after being coated with a joining auxiliary agent ina similar manner.

FIG. 13 shows the AFM measurement result of a predetermined joining face3 ninety-six hours after being coated with a joining auxiliary agent ina similar manner.

FIG. 14 is a perspective view of one set of a first test piece T₂₁ and asecond test piece T₂₂ for ultrasonic welding in a separate state.

FIG. 15 is a cross-sectional view of the same at welding.

FIG. 16 is a perspective view showing a state in which a shearing testis carried out for the respective joined test pieces T₂₁ and T₂₂.

FIG. 17 is a cross-sectional view along the line X-X in FIG. 16.

FIG. 18 is a view showing the results of a shearing strength test of atest piece T₂ in the case where the joining auxiliary agent of Example 1was used and the case where the joining auxiliary agent was not used.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereunder, as an embodiment of the present invention, a joiningauxiliary agent according to the present invention and a method ofinjection molding of a test piece T₁ with a shape shown in FIG. 1 whilejoining with a polyamide resin with the used of the joining auxiliaryagent will be described. As shown in FIG. 1, a test piece T₁ is a testpiece with a uniform thickness and a dumbbell shape in the plan view,which is obtained by subjecting one side of an existing molded articleportion T₁₁ with a shape, in which the dumbbell shaped T₁ is dividedinto two pieces in the longitudinal direction, to injection molding inadvance, coating a predetermined joining face 3, which is the end faceof its narrowed portion, with a joining auxiliary agent according to thepresent invention, placing T₁₁ in a metal mold M shown in FIG. 2 andFIG. 3, then, subjecting an additional molded portion T₁₂, which is theremaining shaped portion of the test piece T₁, to additional injectionmolding.

The metal mold M is composed of an upper mold 5 and a lower mold 6, andthe upper mold 5 and the lower mold 6 have a recess 5 a and a recess 6 acorresponding to the shape with about a half of the thickness of thetest piece T₁, respectively. By bringing the contact faces into closelycontact with each other in a state in which both recesses 5 a and 6 aare allowed to face each other in the vertical direction, whereby acavity corresponding to the shape of the test piece T₁ is formed. Theexisting molded article portion T₁₁ is clamped by the upper mold 5 in astate in which it has been placed in the recess 6 a of the lower mold 6,and a cavity J corresponding to the shape of the additional moldedportion T₁₂ is formed in the remaining space of the cavity. Into thecavity J, a melted resin of nylon 6 is injected, the additional moldedportion T₁₂ is joined to the predetermined joining face 3 of theexisting molded article portion T₁₁. Incidentally, the numeral 7 in FIG.2 and FIG. 3 is an injection gate for injecting the melting resin in theinside of the cavity J.

The existing molded article portion T₁₁ and the additional moldedportion T₁₂ have the same shape, and both have dimensions of length(L₁)×width (W₁)×thickness (K₁)=75×10×3 mm. In the state in which bothare joined together, the test piece T₁ with the total length of 150 mmis formed. In addition, with regard to a law material of a polyamideresin for molding the existing molded article portion T₁₁ and theadditional molded portion T₁₂, as for a nylon 6 resin, Novamid 101, 3CH5(manufactured by Mitsubishi Engineering-Plastics Co., Ltd) was used. Asfor nylon 66, Novamid 3010SR (manufactured by MitsubishiEngineering-Plastics Co., Ltd) was used.

The molding conditions of injection molding are as follows. As a moldingmachine, Niigata Tekko NN50 Hypershot 3000 was used, and the moldingtemperatures were set to 231, 230, 225 and 225° C. in the order from thenozzle portion for injecting nylon 6 (Novamid 101, 3CH5). As for nylon66 (Novamid 3010SR), they were set to 280, 275, 275 and 270° C. in thesame molding machine. In any case, with regard to the temperature of themetal mold, 85° C. was defined as a set value.

The evaluation method of injection molded articles is as follows. Thetest piece T₁ whose sample number N was set to 5 left for 48 hours in atemperature controlled room in which the room temperature was 23° C. andthe humidity was 50%, then, for all the Examples and ComparativeExamples, a tensile test was carried out in the same temperaturecontrolled room by using an instron type universal testing machine,“INSTRON 4505” (manufacture by INSTRON Inc). The test speed was set to50 mm/min, and the maximum tensile strength until the test piece T₁ wasruptured was measured.

In FIG. 4 to FIG. 6, the components of the joining auxiliary agents ofExamples 1 to 33 and the experimental results for the test piece T₁using the respective joining auxiliary agents are shown, and in FIG. 7,Comparative Examples 1 to 4 are shown. In FIG. 4 to FIG. 6, as for anorganic solvent, the molecular weight of methanol is 32.04, themolecular weight of isopropyl alcohol is 60.10, the molecular weight ofacetonitrile is 41.05 and the molecular weight of acetone is 58.08.

In addition, the details of the test conditions and the test results forComparative Example 1, Comparative Example 2, Comparative Example 3 andComparative Example 4, which have the same shape as the above-mentionedtest piece T₁ shown in FIG. 7 are as follows.

COMPARATIVE EXAMPLE 1

A test piece the entire of which was molded in the same shape as that ofthe test piece T₁ by subjecting a nylon 6 resin to a single injectionmolding process under the same molding conditions as those for the testpiece using the joining auxiliary agent was used. As the test results,the tensile strength was 75.1 MPa and the standard deviation was 0.3MPa.

COMPARATIVE EXAMPLE 2

A test piece obtained by performing joining while the additional moldedportion T₁₂ was subjected to additional injection molding with a nylon 6resin without coating the predetermined joining face 3 of the existingmolded article portion T₁₁ with the joining auxiliary agent under thesame molding conditions as those for the test piece using the joiningauxiliary agent was used. As the test results, the tensile strength was17.3 MPa and the standard deviation was 9.8 MPa.

COMPARATIVE EXAMPLE 3

A test piece the entire of which was molded in the same shape as that ofthe test piece T₁ by subjecting a nylon 66 resin to a single injectionmolding process under the same molding conditions as those for the testpiece using the joining auxiliary agent was used. As the test results,the tensile strength was 78.7 MPa and the standard deviation was 1.6MPa.

COMPARATIVE EXAMPLE 4

A test piece obtained by performing joining while the additional moldedportion T₁₂ was subjected to additional injection molding with a nylon66 resin without coating the predetermined joining face 3 of theexisting molded article portion T₁₁ with the joining auxiliary agentunder the same molding conditions as those for the test piece using thejoining auxiliary agent was used. As the test results, the tensilestrength was 6.86 MPa and the standard deviation was 1.6 MPa.

Each of the joining auxiliary agents of Examples 1 to 7 is composed ofone type of organic solvent capable of dissolving a polyamide resin andone type of compound (1), which cleaves a hydrogen bond in a polyamideresin molded article while assisting the dissolution thereof. Each ofthe joining auxiliary agents of Examples 14 to 17 is composed of similartwo types of organic solvents and similar one type of compound (1). Aresin for molding is a nylon 6 resin in any case. As the test results,the resins are joined with a joining strength equal to the materialstrength of an integral article of a polyamide resin (ComparativeExample 1), and with an overwhelmingly superior joining strength to thatof Comparative Example 2.

Next, each of the joining auxiliary agents of Examples 8 to 10 and 17 to22 comprises a polyamide resin in addition to an organic solvent capableof dissolving a polyamide resin and a compound (1), which cleaves ahydrogen bond in a polyamide resin molded article while assisting thedissolution thereof. The difference in the joining auxiliary agentsbetween Examples 8 to 10 and Examples 17 to 22 is that while the formercomprises one type of organic solvent, the latter comprises two types oforganic solvents. A resin for molding is a nylon 6 resin in any Example.As the test results, the resins are joined with a joining strength equalto the material strength of an integral article of a polyamide resin(Comparative Example 1), and with an overwhelmingly superior joiningstrength to that of Comparative Example 2. In addition, as is clear fromthe fact that, for each of the joining auxiliary agents of Examples 1 to7, the tensile strength is increased and the standard deviation becomeslow, the credibility of the joining strength can be increased.

In addition, each of the joining auxiliary agents of Examples 13, 24 to30, 32 and 33 comprises a compound (2), which maintains the action ofcleaving a hydrogen bond of a compound (1), which cleaves a hydrogenbond in the above-mentioned polyamide resin molded article, in additionto an organic solvent capable of dissolving a polyamide resin, thecompound (1), which cleaves a hydrogen bond in a polyamide resin moldedarticle while assisting the dissolution thereof and a polyamide resin. Aresin for molding is a nylon 66 resin in only Examples 32 and 33, and isa nylon 6 in all the rest of Examples. As the test results, the resinsare joined with a joining strength equal to the material strength of anintegral molded article of a polyamide resin (Comparative Examples 1 and3), and with an overwhelmingly superior joining strength to that ofComparative Examples 2 and 4. In addition, for each of the joiningauxiliary agents of Examples 1 to 7, the credibility of the joiningstrength can be increased.

In addition, any of the joining auxiliary agents of Examples 14 to 22contains two types of organic solvents and one type of compound (1),however, while the agents of Examples 14 to 16 do not contain apolyamide resin, the agents of Examples 17 to 22 contain a polyamideresin, which is a different point. With regard to the agents of Examples17 to 22, which contain a polyamide resin, the tensile strength isincreased and the standard deviation is decreased compared to that ofExamples 14 to 16, which do not contain it. In terms of these points,the effect of a polyamide resin as a composition of a joining auxiliaryagent has been proved.

In addition, FIG. 8 shows the results of applying a joining auxiliaryagent, performing molding at every predetermined time interval andperforming a test. The dashed line indicates the change for the agent ofExample 18, which does not contain a compound (2), showing that thejoining strength decreases as time goes on. On the other hand, the solidline indicates the change for the agent of Example 26, which contains acompound (2), showing that the joining strength is maintained withoutdecreasing even as time goes on.

Subsequently, with reference to figures, the events in the presentinvention will be described in order. As shown in FIG. 1, the existingmolded article portion T₁₁, which has been injected and molded inadvance, is coated with, the joining auxiliary agent of Example 23 hereon the predetermined joining face 3 corresponding to the end face havinga width of W and a thickness of K, which is a narrowed portion thereof.Soon after the coating, the joining auxiliary agent slightly dissolvesthe predetermined joining face 3 and loosens the crystal of the nylon 6.Then, the hydroxyl group of added 1,3-dihydroxybenzene and the hydroxylgroup and the carboxyl group of 3,5-dihydroxybenzenecarboxylic acidcatch the bonds of the hydrogen bonds in the loosened nylon 6 resin. Atthis time, the benzene rings of 1,3-dihydroxybenzene and3,5-dihydroxybenzenecarboxylic acid are hydrophobic nonpolar groups,which have an affinity for a methylene chain of the nylon 6, therefore,they are mutually attached automatically, whereby the portions of thehydroxyl groups can be conveniently arranged on the surface of thepredetermined joining face 3.

The state in which the predetermined joining face 3 is slightlydissolved as described above can be confirmed by measuring it with anAFM on a 5-μm scale. Here, FIG. 9 shows the AFM measurement result ofthe surface of the predetermined joining face 3 (before being coatedwith a joining auxiliary agent), and FIG. 10 shows the one after thepredetermined joining face 3 was coated with only methanol. As shown inthe figures, the noise portions (convexes that cannot be measured on a5-μm scale because they are too large) shown to be in a sharp shape suchas two triangular portions and the like in the upper left end region ofFIG. 9 have disappeared in FIG. 10, which indicates that nylon 6 of thepredetermined joining face 3 was dissolved with methanol. In addition,in the joining auxiliary agent, 1,3-dihydroxybenzene and3,5-dihydroxybenzenecarboxylic acid are dissolved other than methanoland isopropyl alcohol, which are organic solvents, and these compoundspromote the dissolution of the predetermined joining face 3. Methanoland isopropyl alcohol in the joining auxiliary agent evaporates soonafter being applied, however, 1,3-dihydroxybenzene remains for a whilewithout evaporating, and moreover, 3,5-dihydroxybenzenecarboxylic acid(CAS number: RN (99-10-5)) does not evaporate, therefore, the amorphousstate of the surface of the polyamide resin of the predetermined joiningface can be maintained for a long time. Accordingly, even if anotherpolyamide resin to be joined and the polyamide resin of the originalmolded article to be joined are not necessarily subjected to a joiningtreatment soon after applying the joining auxiliary agent, the joiningstrength of the both polyamide resins is not lost, therefore, they canapply to a variety of joining operation steps.

The state in which the predetermined joining face 3 is deformed asdescribed above has been confirmed by measuring it with theabove-mentioned AFM. FIG. 11 to FIG. 13 show the AFM measurement resultsof the predetermined joining face 3, 5 minutes, 24 hours and 96 hoursafter being coated with the joining auxiliary agent, respectively. Asshown in the figures, with regard to the surface of the predeterminedjoining face 3, 5 minutes after the coating, the above-mentioned noiseshown in FIG. 9 has disappeared and has been deformed into a less unevensurface, and as time goes on, for example after 24 hours and 96 hours,the dissolution has proceeded, whereby the above-mentioned unevennesshas become further smooth. In this way, it could be confirmed that theaction of the agent that continues to act on the predetermined joiningface 3 in a dry state, while changing the partner resin for a hydrogenbond, dissolves and deforms the predetermined joining face 3 lasts for along time to a certain degree.

In addition, the crystalline nature of the polyamide resin of thepredetermined joining face 3 and the association state of the amidebonds can be confirmed by the infrared absorption spectra. In general,with regard to the infrared absorption spectrum of a polymer, thecrystalline degree can be deduced by finding the absorption regionspecific to the crystalline region and the absorption region specific tothe amorphous region. The absorption of the crystalline nature of nylon6 is located at 1030 and 936 cm⁻¹, and the respective absorptions ofamorphous form corresponding to these are located at 990 and 1139 cm⁻¹in this order. Only the increase and decrease in the spectra for thepredetermined joining face 3 of the existing molded article portion T₁₁coated with the joining auxiliary agent of Example 23 before and afterthe coating over an about 3-day period were plotted out and analyzed. Asa result, in the vicinity of 1000 cm⁻¹, while the absorption linesindicating the crystalline nature decrease, the absorption linesindicating the amorphous form increase instead, which occurs in pairs,and in the vicinity of 936 cm⁻¹, while the absorption lines indicatingthe crystalline nature decrease, the absorption lines at 1139 cm⁻¹,which increases instead, which occurs in pairs, by being made to beamorphous, can be confirmed.

In addition, the characteristic absorptions of the amide bond are theabsorption of amide I caused by the carbonyl group and the absorptioncaused by N—H. The respective absorptions indicating the associationstate are located at 1650 cm⁻¹ and 3360 cm⁻¹, respectively, and theabsorptions indicating the non-association state corresponding to theseare located at 1690 cm¹ and 3420 cm⁻¹, respectively. As a result ofperforming the analysis by the infrared absorption spectra in the samemanner as above, a pair of absorptions in which the absorption at 1650cm⁻¹ decreases and the absorption at 1690 cm⁻¹ showing non-associationincreases instead has been confirmed, and in the case of N—H, a similarpair of absorptions have been confirmed.

Subsequently, when polyamide resin molded articles are joined byultrasonic welding, the joining strengths between the case where thejoining face is coated with a joining auxiliary agent according to thepresent invention and the case where it is not coated with it werecompared and a confirmation test of the joining effect of the joiningauxiliary agent according to the present invention on the ultrasonicwelding were carried out as follows, which will be described. A testpiece T₂ is composed of one set of a first test piece T₂₁ and a secondtest piece T₂₂, and the one with the shape shown in FIG. 14 and FIG. 15was used. The first test piece T₂₁ has a plate form, in which arectangular frame-like protruding portion 22 is integrally formed on arectangular substrate portion 21 and the surface of the protrudingportion 22 became the joining face 23, and the second test piece T₂₂ hasa plate form and the same shape as the rectangular substrate portion 21of the first test piece T₂₁ and its surface became the joining face 24.Both of the substrate portion 21 of the first test piece T₂₁ and thesecond test piece T₂₂ have dimensions of length (L₂)×width(W₂)×thickness (K₂)=25×17×3 mm. The outer dimensions (L₂×W₃) and theinner dimensions (L₃×W₄) of the protruding portion 22 of the first testpiece T₂₁ are (25×12.6 mm) and (20.4×8 mm), respectively, and thethickness (K₃) of the above-mentioned protruding portion 22 is 1 mm.Incidentally, the respective joining faces 23 and 24 of the respectivetest pieces T₂₁ and T₂₂ were in a state without modification after beingmolded and were not subjected to a surface-roughening process (processfor making a joining face rough).

Then, the entire surface of the joining face 23 of the first test pieceT₂₁ was coated with the joining auxiliary agent of the above-mentionedExample 1 of the present invention, thereafter, the joining face 23 ofthe first test piece T₂₁ and the joining face 24 of the second testpiece T₂₂ were brought into close contact with each other. Then,vibration was applied by placing an oscillator 25 of an ultrasonicvibrator on the upper side of the second test piece T₂₂, wherebyultrasonic welding was carried out. The ultrasonic vibrator used herewas “8300, manufactured by BRANSON” and the welding time and weldingstrength were 0.4 sec and 392 N, respectively.

In addition, as shown in FIG. 16 and FIG. 17, the outsides of therespective first and second test pieces T₂₁ and T₂₂ joined as above werecovered with separate tension members 26 and 27, respectively, and thejoined respective test pieces T₂₁ and T₂₂ were housed in their recesses26 a and 27 a, respectively. The respective tension members 26 and 27are connected with a connecting member (not shown) so as not to beseparated in a state in which the respective tension members 26 and 27are allowed to slide in the direction of pulling them. On the oppositefaces of the respective tension members 26 and 27, gripping pieces 26 band 27 b for being gripped by the gripping portion of a tensile testingmachine are integrally provided in a protruding manner, respectively. Inthis way, the respective gripping pieces 26 b and 27 b of the respectivetension members 26 and 27 are gripped by the gripping portion of thetensile testing machine, and when one side thereof is fixed and atensile strength was applied to the other side, a “shearing strength”acts on the joining portion S of the respective first and second testpieces T₂₁ and T₂₂, and by measuring this “shearing strength”, thejoining strength was determined. This “shearing strength” acts on thevertical direction to the paper in FIG. 17.

In addition, as the tensile testing machine, above-mentioned instrontype universal testing machine, “INSTRON 4505” was used. The test wascarried out 5 times on the respective samples by using theabove-mentioned test piece T₁ under the same conditions with regard tothe nylon 6 resin and the nylon 66 resin, and the cases where thejoining auxiliary agent of Example 1 of the present invention was usedand was not used, whereby the “shearing strength” of the respectivefirst and second test pieces T₂₁ and T₂₂, which had been joined wasmeasured. The measurement test results are shown in FIG. 18. In anytest, the “sheared (ruptured) region” of the respective joined first andsecond test pieces T₂₁ and T₂₂ was not the joining portion S, but eitherone of the test pieces T₂₁ and T₂₂. Here, as the nylon 6 resin, Novamid101, 3CH5 (manufactured by Mitsubishi Engineering-Plastics Co., Ltd) wasused, and as the nylon 66 resin, Novamid 3010SR (manufactured byMitsubishi Engineering-Plastics Co., Ltd) was used. Incidentally, in thecase where the joining auxiliary agent of the present invention was notused, the joining was not sufficient at a welding time of 0.4 sec,therefore, the welding time was changed to 0.8 sec.

In the case of the nylon 6 resin, when, for example, the joiningauxiliary agent of Example 23 was used, the “average shearing strength”is 17.8 MPa, however, when it was not used, the “average shearingstrength” is 3.4 MPa, and a more than 5-fold increase in “shearingstrength” could be confirmed. In a similar manner, in the case of thenylon 66 resin, by using the joining auxiliary agent of Example 1 of thepresent invention, a more than 12-fold increase in “shearing strength”could be confirmed. It is understood that the theory of the increase inthis “shearing strength (joining strength)” is substantially the same asthe case of the test piece T₁ which had been subjected to additionalmolding by injection molding as described above. In addition, even byusing a joining auxiliary agent of another Example of the presentinvention, it is assumed that the joining strength is dramaticallyincreased in the same manner as using the joining auxiliary agent ofExample 23, though there is a difference in the increasing ratio of thejoining strength.

1. A joining auxiliary agent for a polyamide resin with which apredetermined joining face of a polyamide resin molded article is coatedto ensure the joining strength between the predetermined joining faceand another polyamide resin when they are joined together, characterizedin that a compound (1), which cleaves a hydrogen bond in the polyamideresin molded article while assisting the dissolution of the polyamideresin, is contained in an organic solvent capable of dissolving thepolyamide resin.
 2. The joining auxiliary agent for a polyamide resinaccording to claim 1, characterized in that the polyamide resin isdissolved and contained in the joining auxiliary agent.
 3. The joiningauxiliary agent for a polyamide resin according to claim 1,characterized in that a compound (2) which maintains the action ofcleaving a hydrogen bond of the compound (1) which cleaves a hydrogenbond in the polyamide resin molded article, in the joining auxiliaryagent.
 4. The joining auxiliary agent for a polyamide resin according toclaim 1, characterized in that the organic solvent in the joiningauxiliary agent is an organic solvent having a molecular weight of 120or less.
 5. The joining auxiliary agent for a polyamide resin accordingto claim 1, characterized in that the organic solvent is a mixed solventof organic solvents composed of plural types thereof.
 6. The joiningauxiliary agent for a polyamide resin according to claim 1,characterized in that the compound (1) which cleaves a hydrogen bond inthe polyamide resin molded article is 1,3-dihydroxybenzene (CAS number:RN (108-46-3)) or/and 3,5-dihydroxybenzenecarboxylic acid (CAS number:RN (99-10-5)).
 7. The joining auxiliary agent for a polyamide resinaccording to claim 2, characterized in that the polyamide resin moldedarticle is nylon 6 or nylon 66, and the polyamide resin contained in thejoining auxiliary agent is identical to the polyamide resin moldedarticle.
 8. The joining auxiliary agent for a polyamide resin accordingto claim 3, characterized in that, in the joining auxiliary agent,1,3-dihydroxybenzene (CAS number: RN (108-46-3)) is contained as thecompound (1) which cleaves a hydrogen bond in the polyamide resin moldedarticle, and the compound (2) which maintains the action of cleaving ahydrogen bond of 1,3-dihydroxybenzene (CAS number: RN (108-46-3)) is3,5-dihydroxybenzenecarboxylic acid (CAS number: RN (99-10-5)) and,or/and salicylic acid (CAS number: RN (69-72-7)).
 9. The joiningauxiliary agent for a polyamide resin according to claim 4,characterized in that the organic solvent is at 50% or higher and 90% orlower by weight.
 10. The joining auxiliary agent for a polyamide resinaccording to claim 6, characterized in that the compound (1) is at 10%or higher and 50% or lower by weight.
 11. The joining auxiliary agentfor a polyamide resin according to claim 7, characterized in that thepolyamide resin is at 0.005% or higher and 1.000% or lower by weight.12. The joining auxiliary agent for a polyamide resin according to claim5, characterized in that the organic solvent comprises two types oforganic solvents, and the mutual ratio by weight is 0.01 or higher and100 or lower.
 13. The joining auxiliary agent for a polyamide resinaccording to claim 8, characterized in that the mutual ratio by weightof 1,3-dihydroxybenzene (CAS number: RN (108-46-3)) and3,5-dihydroxybenzenecarboxylic acid (CAS number: RN (99-10-5)) is 0.001or higher and 1000 or lower.
 14. A joining method using a joiningauxiliary agent for a polyamide resin according to claim 1.